LiNbO_3:Fe晶体折射率调制度的实验研究

Experiment Research of Refractive Index Modulation for LiNbO3:Fe Crystal

利用双光束干涉在不同掺Fe浓度的LiNbO3晶体中分别用红光和绿光写入光子晶格,研究了晶格周期Λ、掺Fe浓度、光波波长λ与光致折射率调制度Δn之间的关系.实验结果表明:用红光(λ=633 nm)写入光子晶格,掺Fe浓度不大于0.05 wt%,或用绿光(λ=532 nm)写入光子晶格,掺Fe浓度不大于0.03 wt%时,LiNbO3晶体的光致折射率调制度Δn随晶格周期Λ的变化存在一个极大值,并随掺铁浓度的增加,相应的折射率调制度极大值也增大,并且向晶格周期小的方向移动,这与一中心模型理论得到的关系曲线的变化趋势相一致;若掺Fe浓度分别大于以上两值时,理论曲线与实验曲线完全不符,说明一中心模型的空间电荷场理论只适用于掺Fe浓度小的LiNbO3晶体.实验还发现LiNbO3:Fe晶体折射率调制度Δn与入光波长λ有关,对同一块晶体,绿光写入时折射率调制度比红光更大,且极大值向晶格周期小的方向移动也更大.

The red and green lights are written into the LiNbOa crystal with different Fe thickness by the double light beam intervene method. The relations of the crystal lattice period,adulterates Fe thickness and light wave wavelength with the refractive index modulation are studied. The experiment result indi- cates： （1） while the red light （,1= 633 nm） is used to induce the photo crystal lattice,the adulterate Fe thickness is not greater than 0. 05 wt %; while it is not greater than 0. 03 wt ~ for green light （,1=532 nm）. （Z） there exists an extremely large value for the refractive index modulation of LiNbO3 and this large value increases when the adulterating Fe thickness is increased. In addition,the change trend of the large value is along the direction to small crystal lattice period,which is in consistent with the one-cen- ter-model theory. （3） if the adulterate Fe thickness is greater than the above values,the theoretical result does not agree with the experiment data completely,which shows that the space charge field theory of one- center-model only applies to the LiNbO3 crystal of little adulterating Fe thickness. The experiment also dis- covers that the refractive index modulation of LiNbO3 ： Fe crystal is related to the inducing light wave- length ,for example,the refractive index modulation in green light is greater than that in red light,and the corresponding extremely large value has greater change compared to the case in red light when using the same crystal.